The present invention relates to the biomedical arts, particularly implantable nerve cuffs for both stimulating and monitoring nerve activity. The present invention finds application in electrodes embedded in nerve trunks and other small tissue strands and will be described with reference thereto. It is to be appreciated that the invention is also applicable to medicinal infusers and other implanted biomedical devices for introducing, monitoring, or removing matter or energy.
Functional electrical stimulation of the nervous system has been shown in recent years to offer great hope in restoring some degree of lost sensory and motor function in stroke victims and individuals with spinal cord lesions. Ways in which functional electrical stimulation can be utilized to restore a particular function include:
(1) the use of surface electrodes to activate the nerves in the general region of interest; PA1 (2) the use of intramuscular electrodes, also to activate the nerves in a general region; PA1 (3) the use of nerve cuff electrodes placed around specific nerves of interest and used to activate them specifically; and, PA1 (4) the use of regeneration-type neural interfaces including microelectrode arrays.
The third alternative offers advantages over the first two in that it requires the least amount of stimulating current and hence the least amount of charge injected into the tissue itself. Because the use of nerve cuff electrodes requires delicate surgery and may damage the nerves, they are usually contemplated when excitation of specific, isolated muscles is desired, or when unidirectional propagation action potentials is required.
The prior art cuff electrodes were of either the split-cylinder type or self-curling coil type. The split-cylinder type included a cylinder of dielectric material defining a bore therethrough of sufficient diameter to receive the nerve trunk to be electrically stimulated. The cylinder had a longitudinal split or opening for receiving a nerve. During installation, the longitudinal split was sutured or otherwise held closed. Although suturing held the cuff in place, an electric current path was defined through the split which permitted current leakage. Also, the suture holding the cuff closed interfered with full expansion of the cuff to accommodate swelling of the nerve. Two or three annular electrodes were positioned on the inner surface of the bore for use in applying the electrical stimuli. The electrical stimuli, for example, may provide functional electrical stimulation, may block natural nerve impulses travelling along the nerve trunk, or the like.
The self-curling type prior art cuff electrodes included a self-curling sheet of non-conductive material biased to curl into a tight spiral. A pair of conductive strips are disposed on the self-curling sheet extending peripherally around the diameter of the cuff passage. The conductive segments may be electrically conductive for applying electrical impulses or fluid conductive for infusing medications. In use, a first edge of the self-curling sheet is disposed adjacent a nerve trunk which is to receive the cuff therearound. The self-curling sheet is permitted to curl around the nerve forming an annular cuff. The effectiveness of this type of cuff is limited due to the placement of the conductive surface on the nerve rather than within the nerve.
Another prior art approach to electrical stimulation of the nervous system is a bipolar, intrafascicular electrode which penetrates the perineurium and resides within an individual fascicle of a nerve. A bipolar electrode pair is formed of two small insulated wires. The insulation is removed from the distal end of each wire. Next, the distal end of one wire is attached to an electrochemically sharpened Tungsten needle. A second wire with similar impedance is aligned with the first wire, and the two wires are threaded into a silastic tube together with a 6-0 silk thread. A small suture loop is left emerging from the distal end of the tubing to allow anchoring of the implant to the nerve. Silastic adhesive is injected into the tubing and allowed to cure. The Tungsten needle is used to thread a first (inside) wire of the electrode through the nerve fascicle for about 1 cm with the exposed zone entering this region. The second (outside) wire is not threaded through the nerve. The needle is then cut off. The other (outside) wire is placed on the outside of the fascicle, and the distal ends of the two wires are fastened to the fascicular endoneurium with a suture. The proximal end is secured in place by suturing the loop emerging from the silastic tubing to the epineurium. The tubing is led to the skin and the wound is closed, leaving the two wires accessible. This method is highly invasive and can permanently damage the nerve through penetration of the perineurium.
The basic idea behind regeneration-type neural interfaces is to manufacture a thin diaphragm with many small holes that can be positioned between the cut ends of a peripheral nerve. The nerve is left to regenerate over time through the many small holes in the diaphragm. The holes are formed by mechanical or laser drilling, or by semiconductor fabrication techniques including wet and dry etching of silicon substrate. Sophisticated interfaces include active electronics on the devices. However, although these devices are theoretically attractive, actual functionality falls short primarily because the nerves tend to regenerate around the interface rather than through it.
The present invention contemplates new and improved cuff electrodes which combine the best features of the above intrafascicular and cuff type prior art methodologies in novel ways. The improved cuffs of the present invention are readily installed without damaging the nerve trunk or other surrounding tissue.